Many types of hollow-fiber blood oxygenators have been or are presently available. These are illustrated by:
1. longitudinal (axial) flow through an annular bundle-see U.S. Pat. No. 4,975,247; PA1 2. circumferential flow around an annular bundle-see U.S. Pat. No. 3,794,468; PA1 3. transverse flow across a bundle of substantially rectangular cross section-see U.S. Pat. No. 5,188,801; and PA1 4. radially outward flow through an annular bundle-see U.S. Pat. No. 3,422,008.
Methods for manufacturing the annular shaped hollow fiber bundles are also available. U.S. Pat. Nos. 3,422,008 and 4,975,247 show a method for spiral winding of ribbons of fibers in a criss-cross pattern. The latter patent shows the formation of regional mats and interleaving of the ribbons at the boundary of such regional mats. The above patents disclose the mounting of a core on a rotating mounting member, where the fiber guide traverses reciprocally along a line parallel to the rotational (longitudinal) axis of the mounting member. The above patents further disclose winding fiber onto the core at constant revolutions per minute (RPM) of the mounting member and constant traverse speed of the guide. As the bundle increases in size, i.e. as the circumference of the bundle upon which the fiber is wound increases, the packing fraction of the fiber decreases. In other words, as the bundle extends radially outward relative to the axis of the core, the packing fraction decreases.
For the purposes of this application, packing fraction is defined to mean the fraction of a unit volume of bundle space occupied by hollow fiber. The packing fraction may be determined in ways known in the art including the convenient method of measuring the interstitial space between fiber by weight gain when a unit volume of bundle is primed with a known liquid. Packing fraction at a particular region or zone located radially outward may be determined by stopping the winding process at the radially inner radial boundary of the region or zone and determining the packing fraction at that stage and then continuing the winding process to the outer radial boundary of the region or zone and determining the packing fraction at that stage. Computations known in the art will determine the packing fraction of the region or zone using the prior two values.
It is an object of this invention to construct a radial flow-annular bundle blood oxygenator where the packing fraction approaches or exceeds the value of about sixty percent for high gas transfer while avoiding high pressure drops for the blood across the bundle and while avoiding clotting of the blood in the bundle. It is a further object of this invention to construct a radial flow-annular bundle blood oxygenator having the above stated objective, combined with the use of a relatively small diameter core in the range of about 0.25 inch to 2.5 inches. Smaller diameter cores have the advantage of lowering the prime volume and decreasing the overall size of the blood oxygenator unit, but have the disadvantage that the above prior art winding methods yield exaggerated decreases in the packing fraction as the bundle extends radially outward. In order for a packing fraction to approach or exceed sixty percent throughout the bundle, the high packing fraction near a small core will encourage clotting.